Doppler effect, electromagnetic spectrum and polarisation

Last time

Before we start: last time, in Hooke's law and the Young modulus, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

The siren that changes pitch

An ambulance races towards you with its siren on. The pitch is high. The moment it passes, the pitch drops. The siren did not change. Your measured frequency did, because of the movement. That is the Doppler effect.

The Doppler effect

The Doppler effect is the change in the observed frequency of a wave when the source and the observer move relative to each other. A source moving towards you squashes the wavefronts together, so the wavelength shortens and the observed frequency rises. A source moving away stretches the wavefronts, so the observed frequency falls. It is a property of all waves, so it is used for radar speed guns and for measuring the motion of stars.

The equation

For a source of frequency f s moving at speed v s, and waves of speed v, the observed frequency is f s times v over v plus or minus v s. Use minus in the denominator when the source moves towards the observer, because f rises. Use plus when it moves away, because f falls. A buzzer of eight hundred and forty-six hertz moves at twenty-five metres per second, with sound at three hundred and forty. Towards you: nine hundred and thirteen hertz. Away: seven hundred and eighty-eight.